This invention relates to directional low frequency ring hydrophones and more particularly to small size circumferentially polarized ring hydrophones operable at extremely low frequencies which are well below any axisymmetric mechanical resonances of the ring, such as ring hydrophone having a high sensitivity to sound incident in the axial direction compared with the sensitivity to sound incident in the radial direction over a band of very low frequencies.
Heretofore rings made of a plurality of stave sections have been set forth operable at low frequencies near the resonance for the ring in water. Such devices have been set forth in U.S. Pat. Nos. 3,177,382 and 3,543,059. Each of these patents refer to operation at very low frequencies. However, it is clear that their intended operating region is near the resonant frequency. Furthermore, the primary purpose of these patents is to produce a high power, low frequency source. In carrying out the teaching of the prior art, there is set forth that large diameter rings of 80 inches are required for operation at a frequency of 500 Hz. Such rings were formed of a plurality of individual segments secured together edge-to-edge with an electrical conductor between the surfaces. In order to decrease the diameter of the unit with low frequency operation, the staves have been fluted so that the ratio of mean diameter to thickness is increased so the natural frequency of resonance of the cylinder with a water load is correspondingly decreased. It is noted that in this configuration, the device operates at the natural frequency of resonance with a water load. Even upon forming the cylinder with a plurality of staves, the diameter is still rather large as set forth in the patents listed above. Furthermore, according to the current state of the art regarding the use of such rings, the axial direction is always assumed to be a direction of relatively low response with the maximum response in the radial direction.